The following story happened to me a few Years ago:
My Flight out of Suvarnabumhi was scheduled to depart 10:00 or 10:30 pm on my last "Permitted until" Date (I was on a 90 Day Single O Visa that time.
Checked in to my flight, went through Immigration at around 8:30 PM, got the exit Stamp.
While waiting at the Lounge the message appeared that my flight was delayed substantially, and new Departure announced for 4 AM on the next Day.
In this situation, have I been on Overstay?